Preheat the grill to medium-high heat.
Season both sides of the chicken breasts with salt and black pepper.
Lightly brush the cut sides of the peaches with grapeseed oil.
Grill the chicken for 5–7 minutes per side, or until the internal temperature reaches 165°F (74°C).
Grill the peaches cut-side down for 3–4 minutes until grill marks appear.
Remove the chicken and peaches from the grill and let them rest for several minutes.
In a small bowl, whisk together the coarse mustard, Dijon mustard, honey, apple cider vinegar, salt, and pepper until smooth.
In a large serving bowl, combine the baby spinach, blueberries, red onion, pecans, and goat cheese.
Slice the grilled chicken and peaches, then arrange them over the salad.
Drizzle with the honey mustard dressing or serve it on the side. This Grilled Peach Chicken Salad is best enjoyed immediately while the chicken is still warm.